Chronic pain, commonly defined as pain that lasts at least six months, is a major medical problem worldwide. Currently used treatment options often fail to relieve all of a patient's symptoms and are associated with significant side effects. Some estimates place the burden as high as US$100 billion per year in the United States alone for the effects of chronic pain including lost productivity and medical expenses.
Pain in general is categorized into three types. Nociceptive pain is the central nervous system's reaction to tissue injury. Nociceptive pain is usually time-limited and tends to decrease as the injury heals. In some cases, such as arthritis and cancer pain, it does not necessarily decrease over time. Regardless of cause, nociceptive pain can be long-term and debilitating.
Neuropathic pain arises out of damage to the central nervous system itself, causing signals to be erroneously interpreted as physical pain. This category of pain includes phantom limb pain in amputees and other types of central pain, including allodynia and hyperalgesia. Some studies consider tinnitus, a chronic “ringing of the ears,” to be a form of neuropathic pain caused by nerve damage in the auditory system.
Some patients experience combinations of nociceptive and neuropathic pain; in many cases they are mutually reinforcing.
Psychogenic pain is either entirely psychological or without other adequate known medical cause and is generally rare. But to the patient experiencing psychogenic pain, it is potentially very disabling.
The therapy of first resort for nociceptive pain is typically nonsteroidal anti-inflammatory drugs (commonly known as NSAIDs). This category of drugs includes ibuprofen (ADVIL®, MOTRIN®), naproxen (ALEVE®), and celecoxib (CELEBREX®.). They work by blocking cyclooxygenase enzymes, both COX-1 and COX-2 in the case of traditional NSAIDs (such as ibuprofen and naproxen), and primarily COX-2 in the case of the more recent drugs (such as celecoxib). The inhibition of COX-1 may lead to gastrointestinal side effects, and recently, COX-2 specific inhibitors have been linked to an increased risk of myocardial infarction and stroke. There may also be adverse renal effects in all cases.
In more serious cases of chronic pain, opioids are frequently prescribed. This category of drugs includes morphine, codeine, fentanyl, and oxycodone. While treatment with opioids is frequently successful, serious side effects include sedation, respiratory depression, constipation, development of tolerance, and in many cases, addiction and abuse.
For neuropathic pain, antidepressants and anticonvulsives have been found to be useful. Tricyclic antidepressants such as amitriptylene, imipramine, doxepine, clomipramine and trimipramine are useful in some cases, but have significant side effects including sedation, arrhythmia, and constipation. SSRI antidepressants (fluoxetine, paroxetine, etc.) have not been found to be as effective in most cases. Anticonvulsives such as carbamazepine, gabapentin, and phenytoin also provide relief in some patients, but the side effects of these drugs include sedation, liver and kidney dysfunction, and aplastic anemia.
Antidepressants are also used in the treatment of neuropathic pain with some success. As set forth above, side effects do occur.
All of the above drug therapies are useful in treating various cases of chronic pain, but the side effect profiles and ineffectiveness for some patients may cause drug therapy to be contraindicated in a relatively significant portion of the population needing relief.
Externally applied non-drug therapies such as transcutaneous electrical nerve stimulation (TENS) and acupuncture are inconvenient, as they typically require the participation of a clinician, and may not provide long-term relief for many patients.
Implantable spinal cord and peripheral nerve stimulators are commercially available. Some work has been done with deep brain stimulators, but such devices are generally configured to provide a constant (or repeating intermittent) signal to a portion of the brain, which may lead to deficit in the area being stimulated or adjacent areas, and may also result in side effects. The application of continuous or semi-continuous stimulation does not require a particularly complex device, but more power is required than would be necessary in a device that stimulated only selectively. And there is some risk of habituation with chronic stimulation.
Spinal cord stimulation and to some extent TENS and implantable peripheral nerve stimulation are thought to provide relief consistent with the “gate control” theory of pain, in which signals in large and small nerve fibers interact to provide pain perception. Simplified to some extent, signals in large nerve fibers primarily represent non-painful stimuli such as touch, and tend to activate neurons inhibiting pain. When pain is present, signals in small nerve fibers inhibit the inhibitory neurons, thereby facilitating pain perception. Spinal cord stimulation (SCS), TENS, and peripheral nerve stimulation (PNS) all tend to control the spike rate of neurons in large fibers (which are more easily activated than small fibers), thereby favoring touch over pain. There is also differential activation of afferent nerves (ascending signals) over efferent (descending). SCS in particular is thought to act through inhibition, possibly through descending inhibitory pathways.
Implantable drug pumps are a promising therapy for chronic pain, allowing analgesic therapeutic agents to be targeted directly on the tissue from which the pain arises. However, in many cases, implantable drug pumps that deliver drugs outside of brain targets are ineffective. In addition to having side effects and controlled substance issues, drug pumps require maintenance and may tend to adversely affect the function of surrounding tissue.
Studies of the brain using functional imaging techniques have identified a number of brain regions that are involved in pain processing. These include but are not limited to the anterior cingulate cortex, prefrontal cortex, insular cortex, thalamus and portions of the somatosensory cortex, for example those corresponding to regions of pain (see, e.g., Krause P, Forderreuther S, Straube A. TMS motor cortical brain mapping in patients with complex regional pain syndrome type I. Clin. Neurophysiol. 2006 January; 117(1):169-76. Epub 2005 Dec. 2; and Apkarian A V, Bushnell M C, Treede R D, Zubieta J K. Human brain mechanisms of pain perception and regulation in health and disease. Eur J Pain. 2005 August; 9(4):463-84, both incorporated by reference herein). Accordingly, surgical and ablative techniques, performed peripherally and in the central nervous system, are gaining favor in particularly difficult-to-control cases of chronic pain. Non-surgical resection and ablation can be performed using radiological Gamma Knife and linear accelerator apparatus. Great precision is required, as surgery and ablation are typically irreversible, and any deficits sustained by the patient are in many cases permanent. And even with such great risks, the success of such procedures is uncertain.
Most of the approaches to treating pain set forth above have side effects, and even when the treatment is effective, the side effects can still be disabling to the patient. Moreover, current treatment options are frequently subtherapeutic and fail to resolve the multiple components of pain perception. Accordingly, there is a need for a responsive implantable system capable of deterring, ameliorating the symptoms of, and in some cases the underlying causes of, various chronic pain conditions.